1. Field of the Invention
The present invention relates to an electron gun body for a color cathode ray tube (hereinafter simply referred to as "CCRT"), and more particularly to an electron gun body for a CCRT capable of improving resolution on the periphery of a screen by eliminating astigmatism without using a separate correction electrode.
2. Description of the Prior Art
In a conventional electron gun generally formed as shown in FIG. 1, a beam forming region is provided by cathodes 1 heated by a heater H for ejecting thermoelectrons in accordance with R, G and B electrical signals, a first grid 2 installed to one side of the cathodes 1 for controlling the electron beams from the cathodes 1, and a second grid electrode 3 installed to one side of the first grid 2 for attracting to accelerate the thermoelectrons gathered around the cathodes 1. Also, a first accelerating and focusing electrode 5 and a second accelerating and focusing electrode 6, which are fixed to a third grid 4 to form a main focusing lens for focusing successively-incoming electron beams from the beam forming region, are arranged in in-line type on one side of the second grid 3.
Additionally, a shield electrode (not shown) is fixed to the second accelerating and focusing electrode 6 for blocking to weaken a leakage magnetic field of a deflection yoke.
In connection with the kinds of electron guns, a third grid and a fourth grid for primarily focusing are additionally inserted between the electron beam forming region and the electrodes constituting the main focusing lens to form a preceding focusing lens system, thereby allowing the electron guns to have a multi-focusing type capability that reinforces the focusing effect.
All the above-mentioned electrodes respectively having three electron beam passing holes for permitting the RGB electron beams formed in the cathodes 1 to be passed are welded to be integrally constructed by a pair of bead glasses 7, being distant from one another.
In the conventional electron gun formed as above, once the cathodes 1 are heated by the heater H to eject the thermoelectrons, the electron beams are controlled in the first grid 2 and, simultaneously, are accelerated by the second grid 3 to be narrowly focused and accelerated while passing through the first accelerating and focusing electrode 5 and the second accelerating and focusing electrode 6 which form the main lens system, because of a voltage difference between the electrodes 5 and 6. Successively, the phosphors coated on the inner surface of a panel are excited to be luminous to produce an image on a screen.
The conventional electron gun has the electron beam passing holes perforated in the shape of nearly right circles sequentially from the first grid 2 to the second accelerating and focusing electrode 6, so that the main focusing lens formed by the first and second accelerating and focusing electrodes 5 and 6 becomes an axially-symmetrical circular lens. Therefore, the electron beams passing through the electron beam passing holes are symmetrically focused in conformity with the Lagrange's reflection law when a voltage required for operating the electron gun is supplied to respective electrodes. Then, the circular electron beams when emitted from the electron gun are focused when reaching the center of the screen unaffected by the deflection yoke to form reduced circular electron beam spots.
In other words, the electron beams from the electron gun scan the overall screen by a deflection magnetic field due to the deflection yoke to reproduce the image.
The deflection magnetic field by the deflection yoke deflects the electron beams to fill in the screen and, at the same time, converges the plurality of electron beams to prescribed spots of the screen in the CCRT that ejects the plurality of electron beams. For executing this function, a self convergence system is adopted, in which the electron beams are emitted in the horizontal in-line direction as described above, and the deflection magnetic field generated by the deflection yoke is forced to be an uneven magnetic field having different magnetic field strengths in the center and the periphery (the periphery of the screen).
By means of the magnetic field of the self convergence system, the RGB electron beams automatically converge on the overall screen.
Such a self convergence magnetic field is classified into a pincushion magnetic field being a horizontal deflection magnetic field, and a barrel magnetic field being a vertical deflection magnetic field.
These magnetic fields are respectively constituted by bipolar and quadrupolar components to mainly deflect by the bipolar component after being emitted from the electron gun and to be minutely subjected to the magnetic force by the quadrupolar component, thereby being affected by a diffusion magnetic field lens in the horizontal direction and a focusing magnetic field lens in the vertical direction.
Accordingly, as shown in FIG. 5, almost the same focusing operation both in the vertical and horizontal directions is carried out at the center of the screen unaffected by the deflection magnetic field. Thus, the electron beams form the substantially circular electron beam spots.
However, in the periphery of the screen affected by the deflection magnetic field, the electron beam of the vertical section is intensely focused by the focusing magnetic lens in the vertical direction to be over-focused, and the electron beam in the horizontal direction is diverged by the diffusion magnetic lens in the horizontal direction to be under-focused, thereby inducing a halo phenomenon to degrade resolution.
For this reason, in order to improve the degraded resolution around the periphery of the screen deteriorated by the deflection magnetic field, a technique shown in FIGS. 2 to 4 (which is disclosed in Korean Patent No. 17874) has been proposed.
Here, through holes 8 and 9 are formed in the opposing planes of the first and second accelerating and focusing electrodes 5 and 6 to allow three electron beams to commonly pass them. Upper rims 10 and 11 respectively bent from the outer circumferences toward the through holes 8 and 9 of the first and second accelerating and focusing electrodes 5 and 6 are provided. An inclined extension electrode 12 as shown in FIG. 4 is fixed to the inner portion of the through holes 8 and 9, maintaining a predetermined distance.
The inclined extension electrode 12 is formed by a head portion 13 to be fixed into the first and second accelerating and focusing electrodes 5 and 6, a sloped portion 14 having triangular projections 14a on the upper and lower portions thereof, and a bottom portion 15 having a center hole 15a extending to the sloped portion 14. Here, an inclination angle between the sloped portion 14 and the bottom portion 15 ranges from 100.degree. to 140.degree..
The reason of setting the inclination angle from the head portion 13 to the bottom portion 15 from 100.degree. to 140.degree. is in that the beam spot is the smallest within the above range.
Also, the reason of extending the center hole 15a formed in the inclined extension electrode 12 to the sloped portion 14 is in that the spherical aberration is caused to be decreased to thus minimize the beam spot size.
Briefly, the magnetic field is forced to be consistently formed.
According to the electron gun adopting the inclined extension electrode 12, when the dimensions of the inclined extension electrode satisfy the static convergence, i.e., when the side beam and central beam coincide in the center of the screen, the electric field of the side hole becomes asymmetric in the horizontal and vertical directions due to the projection 14a of the sloped portion 14. Consequently, since astigmatism becomes greater in the side hole, the astigmatism which is a focusing difference in the horizontal and vertical directions cannot be eliminated throughout the screen as shown in FIG. 5.
This is because the electric fields distributed to the center hole and side hole of the main focusing lens are basically different from each other, an additional correction unit is necessarily required.
In addition to this, the molding as well as forming for fabricating the inclined extension electrode 12 become very difficult and exacting, resulting in lower productivity.
Referring to FIG. 6, another technique for improving the above-described problems has been proposed. Here, a correction electrode 17 having horizontal barriers on the upper and lower portions of electron beam passing holes 16a is welded to be fixed to a shield cup 16, and in turn, the shield cup 16 having the correction electrode 17 fixed thereto is inserted to the second accelerating and focusing electrode 6.
This technique is advantageous in that the correction electrode 17 sufficiently blocks the magnetic field produced by the deflection yoke when the electron beams emitted from the cathodes pass through the second accelerating and focusing electrode 6, which can correct the astigmatism in a desired direction without affecting the convergence.
In this technique, however, a punching operation is performed to form the electron beam passing hole 16a during processing of the shield cup 16 to which the correction electrode 17 is fixed. Therefore, it is difficult to flatten a connection plane (i.e., the surrounding portion of the electron beam passing hole 16a) for fixing the correction electrode 17, and match the electron beam passing holes formed in the shield cup 16 and the correction electrode 17. As the result, the welding position of the correction electrode 17 is inaccurate causing a change in the movement path of the electron beam and, furthermore. Impending precise processing for making the upper and lower lengths of the correction electrode 17 be the same, with the result that resolution is degraded.